late 14c., prolongacioun, "condition of being extended;" early 15c. as "protraction, lengthening in time;" from Old French prolongacion (14c.) and directly from Medieval Latin prolongationem (nominative prolongatio), noun of action from past-participle stem of Late Latin prolongare "to prolong, extend," from Latin pro "forth" (see pro-) + longus "long" (adj.); see long (adj.).